Our clinic has, forced by the above problems related to Mexiletine, treated a few patients with the drug Lamotrigine with pronounced positive effect in all. Lamotrigine belongs to the same category of drugs as Mexiletine but has fewer and milder side effects. Based on the similarities of the 2 drugs in pharmacological action and the positive experiences investigators are convinced that Lamotrigine will show a positive effect if evaluated in a broader scale. Due to the advantages of Lamotrigine compared to Mexiletine investigators find it of outmost importance for patients that this drug is assessed formally to establish Lamotrigine as a treatment choice for myotonia. Investigators believe that this will potentially make a huge difference in life quality for persons with myotonia. Investigators aim at investigating the efficacy and tolerability of Lamotrigine in the treatment of myotonia in a randomized doublet blinded placebo controlled crossover study.